To acquire digital data of a large screen, a two-dimensional X-ray sensor (FPD: Flat Panel Detector) for X-ray imaging has been developed recently. In particular, an imaging apparatus using a two-dimensional X-ray sensor having a large light-receiving surface with a size of 43 cm×43 cm has been put in practical use for simple imaging.
Additionally, a CT apparatus for acquiring three-dimensional image data by using a two-dimensional X-ray sensor has been developed. In this CT apparatus, the two-dimensional X-ray sensor receives an X-ray beam that is called a cone beam and has a three-dimensional extent.
When a cone beam is used, the range of patient imaging by scanning of one rotation can widen as compared to a CT apparatus using a fan beam with a two-dimensional extent. This improves the imaging efficiency.
However, it has been pointed out that an increase in the cone angle in the Z-axis direction of X-ray irradiation increases the influence of scattering rays and errors in reconstruction calculations, resulting in degradation of image quality.
A conventional radiographic imaging technique for a still image is disclosed in Japanese Patent Laid-Open No. 2003-209746. In Japanese Patent Laid-Open No. 2003-209746, when the sensor output is saturated, the estimated output in the saturation region is calculated based on the signal in the leading edge or attenuation region of the sensor output before or after the saturation. The imaging apparatus disclosed in this prior art generates image data by combining the steady output and estimated output.
As described above, the CT apparatus using a cone beam can widen the range of patient imaging by scanning within one rotation. For this reason, the number of rotations can be small, and the imaging efficiency can be increased. However, it has been pointed out that an increase in the cone angle in the Z-axis direction of X-ray irradiation increases the influence of scattering rays and errors in reconstruction calculations, resulting in degradation of image quality.
A patient has regions such as the lungs that pass X-rays well and regions such as the belly that does not pass X-rays well. The CT apparatus using a cone beam can hardly change the irradiation dose for each region.
In the method of calculating an estimated output in a saturation or overflow region, as described in Japanese Patent Laid-Open No. 2003-209746, even a slight estimation error in each projection image can largely affect a reconstructed image because of the principle of CT reconstruction.